When I import images as planes it shows a random line at the top
Image textures employ interpolation to blend pixels into the neighbouring space so as to smooth the image rather than provide a hard pixelated effect. With the image set to Repeat there is actually a copy of the image just beyond the image borders. In your case the image against the opposite edge is butter right up against the edge and so the interpolation is causing it to 'bleed' across te border - the bottom of your grass image is bleeding into the top.

To resolve this, simply set the image to Clip rather than Repeat.

The option is located under the "Texture" panel:


If you're using Cycles


If you're using Blender render